The present invention relates generally to diagnosing maladies of the eye using polarized light sources.
Knowing the thickness of a patient""s retinal nerve fiber layer can be crucial in diagnosing glaucoma and other optic nerve diseases. It happens that the nerve fiber layer is xe2x80x9cform birefringentxe2x80x9d, which means that, if the polarization axis of a polarized beam of light passing through the layer is neither parallel nor perpendicular to the nerve fiber bundles, retardation is introduced into the beam. More specifically, birefringence is an optical property that arises from the anisotropy of a medium through which polarized light propagates, and it is manifested by the varying of the velocity of the light, with the velocity depending on the direction of propagation and polarization axis of the light. When light propagates perpendicular to the optic axis of an anisotropic material, two orthogonally polarized components of the light, one with polarization parallel to the optic axis and the other with polarization perpendicular to the optic axis, will travel at different velocities, resulting in a phase shift between the two components. This phase shift is referred to as xe2x80x9cretardationxe2x80x9d. The polarization state of the emerging backscattered light changes based on the amount of retardation between the two components. A retardation map can be generated based on the backscattered light that represents the thickness of the nerve fiber layer and, hence, that is useful for diagnosing maladies of the eye.
Accordingly, the present assignee has disclosed laser diagnostic devices in U.S. Pat. Nos. 5,303,709, 5,787,890, 6,112,114, and 6,137,585 that measure the thickness of the nerve fiber layer by measuring the amount of retardation of laser light in the fiber layer, with the amount of retardation then being correlated to layer thickness in accordance with principles known in the art. Likewise, as understood by the present invention the so-called Henle fiber layer, which includes photoreceptor axons and which has radially distributed slow axes centered about the fovea in the macula of the eye, is also form birefringent and consequently, its thickness also can be measured for diagnostic purposes using laser light.
As further recognized herein, however, portions of the eye (hereinafter collectively xe2x80x9canterior segmentsxe2x80x9d) that are anterior to the nerve fiber layer and Henle fiber layer are also birefringent. For instance, both the cornea and lens are birefringent. Moreover, the axial orientation and magnitude of birefringence of the anterior segments can vary significantly from person to person. Since the diagnostic beam must pass through these anterior segments, the present invention understands that the laser beam retardation caused by these portions must be accounted for, to more accurately map posterior segments such as the nerve fiber layer and Henle fiber layer.
In the above-mentioned U.S. Pat. No. 5,303,709, a corneal compensator was disclosed for neutralizing the effects of the birefringence of anterior segments of the eye on a diagnostic beam meant to measure the thickness of the nerve fiber layer. The compensating structure in the ""709 patent includes a polarization sensitive confocal system attached to a scanning laser retinal polarimeter. The detector of this apparatus includes a pinhole aperture set to be conjugate with the laser source and the posterior surface of the crystalline lens so that only reflected light from the posterior surfaces of the crystalline lens is captured and analyzed. A variable retarder is then set to null any retardation in the returned light beam.
While effective for its intended purposes, the compensating features of the ""709 patent, as recognized herein, require that several optical components be added to the already complex optical system of a scanning laser polarimeter. Moreover, the ""709 invention uses the patient""s lens as a reference surface for determining anterior segment birefringence. As recognized herein, a patient""s lens reflection intensity which is captured by the confocal imaging can fluctuate due to eye movement, and consequently it can be difficult to accurately compensate for anterior segment birefringence when using the lens as a reference surface.
As also recognized herein, apart from the present invention""s understanding that no method has yet been disclosed for compensating for the birefringence of anterior segments of the eye by post-measurement calculations, such post-measurement compensation can be complicated. This is because the particular contribution of the Henle fiber layer to overall retardation is not necessarily known, and instead is mixed in with the overall anterior segment birefringence.
The present invention accordingly recognizes that it would be desirable to provide a method and apparatus for measuring the birefringence of segments of the eye that are anterior to the retina, despite eye movement.
The apparatus and methods disclosed herein overcome the shortcomings of the above-mentioned corneal measurement apparatus. Instead of detecting the polarization state of the reflection from the back surface of the crystalline lens, polarimetry is performed on reflections from the fundus to determine the anterior segment retardation magnitude and axial orientation. The anterior segment birefringence is determined by analyzing the polarization state of the back-scattered light from one of the following fundus regions: the macula, the major retinal vessels, and locations where the retinal birefringence is inconsequential compared to the birefringence of the anterior segment.
To do this, the present invention uses a polarized light beam of known polarization state. One of the two simple polarization states are preferred: a rotating linearly polarized light, and a circularly polarized light. A variable retarder is provided to cancel out the anterior segment birefringence so that the incident beam remains a linearly polarized light beam or circularly polarized light beam when impinging on the fundus.
With this invention, a simplified scanning laser polarimeter can use the same beam path to measure the corneal and lens birefringence as is used to measure the retinal nerve fiber layer birefringence. Also, the anterior segment birefringence can be determined without eye movement interfering with the determination. Moreover, by measuring the anterior segment birefringence along substantially identical beam paths as are used for the measuring beams of retinal nerve fiber layer, a more accurate measurement of the anterior segment birefringence can be made, since corneal birefringence varies with the incidence angle of the beam and with the location of the cornea.
As disclosed in greater detail below, the reference target used for the backscattering of the probe beam to detect birefringence of the anterior segment is not on the lens, but rather is associated with the retina. For example, the target can be the Henle fiber layer in the macula. Alternatively, major retinal blood vessels can be used as the target. This is because, as recognized herein, retinal blood vessels are close to the retinal surface and the specular reflection from the top surface of the major retinal vessels maintains the polarization state of the incident beam. As a consequence, retardation measured at major blood vessels is a measurement of the birefringence from the anterior segment. Fundus regions where the retinal birefringence is at a minimum can also be used as a reference target, because the back-scattered light from these regions preserves the polarization state of the incident light.
The output of the invention is a retardation map of the nerve fiber layer or of the Henle fiber layer (photoreceptor axon layer), which can be used as a tool to diagnose and monitor glaucoma, macular degeneration, optic neuropathy, optic neuritis, aging, and other eye diseases, such as those that cause ganglion cell or photoreceptor axon atrophy.
In one aspect, a method is disclosed for determining a birefringence of a posterior segment of an eye having an anterior segment and a retina. The method includes directing a first beam against a portion of the retina to render a first reflected beam, and based on polarization properties of the first reflected beam, determining a birefringence of the anterior segment. The method further includes configuring a polarization compensating device to null the birefringence of the anterior segment. A second beam is directed through the polarization compensating device and against a portion of the retina to render a second reflected beam. Then, based on polarization properties of the second reflected beam, the method determines a birefringence of the posterior segment.
In a preferred embodiment, the birefringence of the anterior segment is determined by configuring the polarization compensating device to have a null setting, and then directing the first beam through the device to render the first reflected beam. Next, the polarization compensating device is configured to have a non-null setting. The method once again directs the first beam through the device to render the first reflected beam.
As set forth in greater detail below, in one presently preferred embodiment, the birefringence of the anterior segment is determined by determining a maximum magnitude derived from the first reflected beam, determining a Henle fiber layer value based on a difference between the maximum magnitude and a minimum magnitude derived from the first reflected beam, and then determining a retardation value of the anterior segment by subtracting from the maximum magnitude the Henle value and a setting value of the polarization compensating device. An algorithm is also disclosed for determining the birefringence of the anterior segment. The algorithm includes determining a retardation value xcex4 as follows: xcex4=[xcex/360xc2x0]sinxe2x88x921[Imax/Itotal]xc2xd, wherein Imax is a maximum output intensity of a first detector detecting the reflected beam, Itotal is the sum of the intensities output by two detectors detecting the reflected beam, and xcex is the wavelength of the first reflected beam.
The birefringence of the anterior segment alternatively can be estimated as followed. In this embodiment, a slow polarization axis of the anterior segment is observed. The method then determines a magnitude of a retardation of the anterior segment based on an average retardation value taken from a ring area centered on the fovea of the eye within a cone of 6xc2x0 as measured from the pupil of the eye, where the fiber layer is relatively thin.
Circularly polarized light can be used in another embodiment. In this method, the circularly polarized light beam is directed onto the macula. A quarter wave retarder is set to zero, and then the axis of retardation is determined from the below-described xe2x80x9cbow tiexe2x80x9d. The quarter wave retarder axis is aligned with the observed axis and its value increased from zero until the xe2x80x9cbow tiexe2x80x9d disappears from the image. At this point, the axis and value of the retarder represent the axis and value of the anterior segment retardation.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: